1. Field of the Invention
This invention relates to a method for producing a removable metal layer by plating, more particularly, to an improved electro-plating method which is suitable for making an information-recorded matrix or disc used in the process of manufacturing a video disc.
2. Description of Prior Art
In the conventional process of manufacturing a video disc, a stamper, carrying a negative pattern of the video information on its surface, is manufactured by electroplating a metal layer on a metallic, suitably nickel, mother plate. The stamper is separated from the mother plate after electroplating. Various methods have been used in the prior art to facilitate the separation of the stamper from the mother plate. In one method, a nickel coated mother plate is dipped into a 5 to 10 percent solution of potassium bichromate for several tens of seconds at room temperature until the nickel to be plated is oxidized to form a parting layer. The oxidized surface is then electroplated to form the stamper. The parting layer of oxide permits the stamper to be separated from the mother plate. This method is known as the "bichromate method" and is attractive because it requires no special equipment to perform it. The bichromate method is now being replaced by other methods since it has been found to be the source of pollution by hexavalent chromium. Furthermore, the bichromate method produces dissolution and discoloration of the surface of the mother plate when the solution is too strong and fails to form a satisfactory parting layer when the solution is too weak. There are also few chemical oxidizing agents which have sufficient oxidizing power.
According to another prior art method, of forming a parting layer, the mother plate is dipped into a several percent solution of sodium hydroxide and is subjected to anodic oxidation at a current density of 0.5 to 5 A/dm.sup.2 (amperes per decimeter.sup.2) normally at 40.degree. to 60.degree. C., whereby an oxide layer of oxidized nickel is formed as a parting layer. This method is known as "anodic oxidation method" and does not create pollution. This method has disadvantage that a bath for the anodic oxidation must be heated to a an elevated temperature and that special electrical equipment for anodic oxidation is required. This method has the further defect that it is hard to uniformly oxidize the entire surface of the mother plate. In addition, the nickel of the mother plate is partially oxidized and dissolved each time the operation is performed. This degrades precise copying.
In both the bichromate methods, and anodic oxidation method, the oxide parting layer formed on the mother plate may become hydrophobic when dry. The oxide layer accordingly repels water and interferes with succeeding washing and electroplating processes. For this reason, an additional process is required to coat a surface active agent on the surface of the oxide layer on the mother plate before electroplating to form the stamper. This additional operation increases the labor and cost of the process.